Applications using Raman scattering to identify unknown materials are rapidly proliferating, e.g., in the areas of security and safety, biotechnology, biomedicine, industrial process control, pharmaceuticals, etc. This is largely due to the rich and detailed optical signatures made possible by Raman scattering.
In a Raman analyzer, a laser is used to generate a stable and narrow linewidth light signal which functions as the Raman pump. The Raman pump is directed at the specimen, resulting in Raman scattering, which provides a unique Raman signature for the specimen. The specimen's Raman signature can then be used to determine the composition of the specimen.
For portable Raman applications, small size and low electrical power consumption are essential. In this latter regard, it should be appreciated that the laser in a Raman analyzer typically accounts for the majority of the device's power consumption. Hence, in portable Raman units, the laser tends to dominate the battery lifetime of the unit.
Semiconductor lasers are one of the most efficient lasers known. Semiconductor lasers can have wall-plug efficiencies of greater than 50%, which is rare for other types of lasers. However, the semiconductor lasers generally need to be wavelength stabilized (e.g., at 785 nm or other operating wavelengths, depending on the application).
In Raman applications, the most common technique for stabilizing the laser's operating wavelength is to use a diffraction grating in an external cavity geometry. This arrangement can be used to easily and inexpensively stabilize the laser's operating wavelength to a few inverse centimeter (e.g., <50 cm−1). However, such an arrangement tends to be relatively temperature sensitive, i.e., temperature changes can cause thermal expansion of various elements of the assembly, which can in turn detune the alignment and change the laser's operating wavelength and/or vary the laser linewidth. As a result, a thermo-electric cooler (TEC) is commonly used to stabilize the laser's operating temperature to within a couple of degrees Celsius. However, thermo-electric coolers themselves consume substantial amounts of power, making such an arrangement undesirable in portable applications where power consumption is an important consideration.
As a result, in portable applications, it is common to use “uncooled lasers” (i.e., those lacking a thermo-electric cooler); however, these uncooled lasers are only operated over a relatively limited temperature range, in order to meet operating parameters of spectral purity and power consumption.
Thus, there is a need for a low-power, uncooled laser which can provide a stable, narrow-linewidth signal, over a wider operating temperature range.